A digital camera typically includes an auto exposure (AE) function for automatically determining an exposure (see, for example, Japanese Laid-Open Patent Publication No. 2001-086396). Such a digital camera determines an exposure based on, for example, an image captured by an imaging device. Therefore, exposures of first few images (e.g., one to three images) initially captured by the imaging device may not be properly set. For example, in a digital camera including a display for displaying preview images, the first few images are used for AE calculations and are not displayed on the display. That is, this type of digital camera starts to display images only after an appropriate exposure is determined.
For this reason, there have been proposed methods for reducing the time taken by a digital camera to display an image after a user turns on the digital camera. In a proposed method, a digital camera sets an imaging device in a high-speed mode (where pixels are skipped) at start-up, and performs AE calculations while the frame rate is high. The digital camera performs AE calculations for respective frames, and gradually changes the exposure value for the frames until an appropriate exposure value is attained. Then, after the appropriate exposure value is attained through the AE calculations, the digital camera changes the drive mode of the imaging device to a normal full pixel mode, and displays the first image captured in the full pixel mode on a display.
In another method, instead of using a high-speed mode, a digital camera performs “one-shot AE” at start-up. In “one-shot AE”, unless the brightness of an image captured by an imaging device is saturated, an appropriate exposure is determined with “one shot”.
Also, Japanese Laid-Open Patent Publication No. 2008-042248, for example, discloses a digital camera that determines a temporary exposure value at start-up without using an image captured by an imaging device. This digital camera determines a temporary exposure value at start up based on, for example, an electromotive force obtained from electrical charge accumulated on a solid-state imaging device while the power is off.
In the method where a digital camera sets the drive mode of an imaging device to a high-speed mode at start-up, the digital camera switches the drive mode of the imaging device from the high-speed mode to a normal full pixel mode before displaying an image on a display. Accordingly, with this method, changing the drive mode of the imaging device causes a processing delay and increases the time necessary to capture an image with an appropriate exposure. Also, with the method where a digital camera performs “one-shot AE”, when the brightness of an image captured by an imaging device is saturated, the digital camera needs to repeat the “one-shot AE” until the brightness of the image becomes unsaturated. In this case, the repetition of the “one-shot AE” causes a processing delay and increases the time necessary to capture an image with an appropriate exposure.